This invention relates to a method by which products containing organic halides are converted to nontoxic inorganic compounds without evolving highly toxic dioxins, as well as an apparatus for implementing the method.
Volatile organic halides have the potential to deplete the ozone layer whereas nonvolatile organic halides can contaminate the environment such as soil and groundwater. If organic halides are disposed of by incineration, highly toxic dioxins may be evolved from the incinerator. The emission of dioxins has posed a big social problem since they seriously impair the human health by accumulating in the human body and presumably affecting nursing infants via their mother's milk.
The use of PCBs, another highly toxic substance, has been banned but most of them are stockpiled in the absence of an effective and safe way of disposal. The production of volatile organic fluorides have suspended since they cause ozone depletion but even today they are used in large quantities. In spite of these difficulties, halogenated organic compounds have useful characteristics; for example, chlorofluorocarbon are used as coolants, fire retardants and detergents whereas chlorinated compounds such as trichloroethylene, methylene chloride and tetrachloroethylene are used as detergents; most pharmaceuticals, agrichemicals and insecticides contain chlorine.
It is therefore necessary that these substances be used in a controlled fashion and, after use or if their substitutes become available, be converted to a harmless state. In fact, no effective methods or apparatus that meet this need have been developed.
Under the circumstances, it has been proposed to decompose PCBs and dioxins either biologically by using microorganisms or chemically by using supercritical water reactions. Speaking of the apparatus for treating organic halides or their potential generators by incineration, studies and proposals have been made to retrofit the existing incinerators such that they will evolve no dioxins. However, the proposals so far made are comparatively difficult to operate, require expensive apparatus or have the problem of complexity.
Japanese Patent Public Disclosure No. 187645/1997 proposes an improved method for treating harmful organic chlorinated compounds such as virgin PCBs in prolonged storage or spent trichloroethylene and trichloroethane that are stored untreated. In the method, the harmful chlorinated compound is reacted with calcium oxide, barium oxide or a mixed oxide thereof in air or an inert gas at 400-650° C., whereby the chlorine in the organic chlorinated compound (which is the most difficult to treat) is altered to calcium chloride or barium chloride. This method of rendering organic chlorinated compounds harmless by dechlorination reaction remains only theoretical and is far from being out of the glass reactor to be used commercially. For example, transformers with ratings of 50 kV·A use a mixture of PCB and a mineral oil as the insulating oil and the residual gas from the dechlorination treatment by the method contains organic matter but this cannot be treated continuously.
Japanese Patent Application No. 46800/1999 teaches an improved method and apparatus by which products containing organic halides either alone or in admixture with organic compounds are converted to inorganic substances. The products are gasified or atomized and subjected to catalytic reaction with calcium oxide, barium oxide or a mixture thereof that are heated at 400-700° C., whereby the halogen in the organic halide is converted to a calcium halide and/or a barium halide. A moving bed of calcium oxide and/or barium oxide in particulate form is continuously brought into countercurrent contact reaction with the product (feedstock: organic liquid containing organic halides used solvent) and the halogen in the resulting organic halide reacts with the oxide to form a halide which is continuously discharged from the contact reaction zone whereas the residual gas from the contact reaction is contacted with a platinum or palladium catalyst on a ceramic support at 200-500° C. and the residual non-halogenated organic compound is incinerated. In this method, if the particles of calcium oxide and/or barium oxide were moved continuously in a more efficient way to have contact reaction with the feedstock, the need to provide the means for catalytic treatment of the effluent gas could be eliminated and the overall system configuration would become more simplified.